1. Field of the Invention
This invention relates to spray guns, more particularly of the type in which a liquid such as paint is sprayed by the action of compressed air that arrives through the hollow handle of the spray gun to entrain and atomize the liquid.
2. Description of Related Art
The central mechanism in a spray gun is the liquid fluid flow assembly. The assembly conducts the fluid from a fluid container such as a fluid cup, pressure pot, or reciprocating or diaphragm pump. The pressurized fluid, such as paint, flows into a fluid block, generally makes a right angle turn, then flows around the shaft of a needle valve to the nozzle. The flow is controlled by a needle valve, the shaft portion of the needle which transfixes the fluid block and the point portion of the needle which interacts with the nozzle. Retraction of the needle withdraws the point from the nozzle and allows fluid to be sprayed from the nozzle. Engagement of the needle point with the nozzle closes the valve and prevents the flow of fluid.
Three portions of the fluid flow assembly require special maintenance and cleaning. 1. The point of the needle valve and the nozzle with which it interacts must be kept scrupulously clean of caked and dry paint and other dried sprayed materials. Failure to keep this portion clean results in failure of the needle valve to seat properly and leakage of paint from the nozzle tip. 2. The packing which surrounds the needle shaft as it passes into the fluid block must be kept properly tightened and clean of dried paint. Failure to properly tighten the packing results in either prevention of movement of the needle valve or leakage of pressurized paint into the body of the spray gun. Paint in the gun body will mix with atomizing air and will block the air passages in the air cap, causing distorted fan patterns. Paint in the gun body also will cause trigger malfunction. 3. The fluid passage in the fluid block through which the fluid and needle valve shaft passes also must be kept clean or the passage of fluid will be impaired.
Most spray guns do not have a fluid flow assembly which may be removed for cleaning,maintenance or replacement. It is particularly difficult to clean, maintain or replace, and adjust the fluid block packing gland. The packing gland, consisting of one or more gaskets and an adjusting nut, functions to prevent leakage of fluid back past the needle valve shaft into the body of the gun. It is important that the packing gland be adjusted so that it is tight enough to prevent retrograde flow of the pressurized fluid, yet loose enough to allow free movement of the needle valve shaft. Most spray guns often suffer blockage from paint leaked through the packing gland. In addition, failure to properly clean the needle valve point and nozzle often results in leakage of fluid from the nozzle. An unclean needle nozzle also causes the paint to "spit" or squirt, causing unsatisfactory finish and quality of the surface coated. Some spray guns have fluid flow assemblies which may be removed for cleaning. None of the prior art spray guns, however, have such assemblies which are easily removed, allow easy adjustment of movement of the needle valve, and are inexpensive to manufacture.
U.S. Pat. No. 3,796,376 discloses a spray gun having a conventional fluid flow assembly including a needle valve, a stuffing box, and a nozzle. The fluid flow assembly, however, is removable but complicated and difficult to remove.
U.S. Pat. No. 4,817,872 discloses a spray gun having a cylindrical cartridge fluid flow assembly which may be removed for cleaning. The cylindrical assembly has a fitting to which the paint container is attached. The cylindrical assembly fits into the barrel of the gun with the fitting extending through a slot in the barrel. A set screw secures the assembly to the barrel.
None of the prior art spray guns have fluid flow assemblies which are easily removed and cleaned, maintained, or replaced and allow easy adjustment of the packing gland without removal of the assembly, and are simple and inexpensive to manufacture.